Channel fault detection for channel diagnostic Systems

ABSTRACT

A method and computer program product for detecting faults in cables. The invention comprises receiving a first reflected signal; comparing the first reflected signal amplified with a first predetermined receiver gain setting with a first threshold; if the value of the amplified first reflected signal is greater than the value of the first threshold, then terminating detecting; if the value of the amplified first reflected signal is not greater than the value of the first threshold, then comparing a second reflected signal amplified with a second predetermined gain setting different from the first gain setting with a second threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.60/584,173, filed Jul. 1, 2004, entitled “METHODS FOR PERFORMING CHANNELDIAGNOSTICS,” incorporated herein by reference in its entirety.

This invention is also related to the subject matter disclosed in U.S.patent application Ser. No. 10/281,992, filed Oct. 29, 2002, PublicationNo. 20040013208, entitled “METHOD AND APPARATUS FOR DETERMINING ARECEIVER SAMPLING PHASE FOR USE IN DIAGNOSING A CHANNEL,” and commonlyowned with the present invention. The disclosure of the '992 applicationis incorporated herein by reference in its entirety.

BACKGROUND

1. Field of Invention

The present invention relates generally to communication devices andrelated methods, and more particularly, to such a device using anadaptive filter.

1. Related Art

A known communication system includes a pair of transceivers thatcommunicate with each other over a communication channel, such as a wireor optical cable. On occasion, a fault in the communication channelinhibits communication between the transceivers. It is desirable to beable to determine whether such a fault exists. If such a channel faultexists, it may be difficult to determine useful information about thefault upon basic inspection, such as its location in the channel. It isdesirable to be able to determine information about the fault, includingits location in the channel and/or the type of fault that has occurred.It is also desirable to be able to determine other information about thecommunication channel, such as the length of the channel in the absenceof a fault. What is needed is a technique to determine theabove-mentioned information without having to examine the channelphysically.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a method and computer programproduct for detecting faults in cables. The invention comprisescomputing a plurality of thresholds representing a correspondingplurality of values of reflections in the cable. A received reflectionis compared to a given threshold. If the received reflection value isgreater than the given threshold value, then the location of the cablefault is determined. If the received reflection is not greater than thegiven threshold value, then no further action need be taken.

Further embodiments, features, and advantages of the present invention,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and enable a person skilled in the pertinent art to make anduse the invention. In the drawings, like reference numbers indicateidentical or functionally similar elements.

FIG. 1 is a block diagram of an example communication system in whichthe present invention can operate.

FIG. 2 shows a channel diagnostic system using an Adaptive EchoCanceller with programmable gain tap.

FIG. 3 shows a detailed diagram of the Adaptive Echo Canceller.

FIG. 4 shows a graph of the piecewise linear function thresholdsettings.

FIG. 5 shows a data path of resource sharing to compute the PiecewiseLinear Function for each tap threshold.

FIG. 6 shows an open cable fault.

FIG. 7 shows a shorted cable fault.

FIG. 8 is a flow chart of the cable diagnostic method.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those skilled inthe art with access to the teachings provided herein will recognizeadditional modifications, applications, and embodiments within the scopethereof and additional fields in which the invention would be ofsignificant utility.

FIG. 1 is an example communication system 100 in which the presentinvention can operate. System or environment 100 includes atransmitter/receiver (transceiver) assembly 102 and a transceiverassembly 104, that communicate with each other over communicationchannels 106 a-106 n . Transceiver assembly 102 includes transceivers108 a-108 n, coupled respectively to communication channels 106 a-106 n.Transceiver assembly 104 includes transceivers 110 a-110 n respectivelycoupled to communication channels 106 a-106 n. Communication channels106 may include wire and/or optical cables. The terms “channel” and“cable” are used equivalently and interchangeably herein. IEEE standardsrecommend that an Ethernet cable be no more than 100 m in length.However, installers often will use cables up to 150 m in length.

In operation, transceiver 108 a receives an input symbol stream 112Tfrom an external data source, not shown. Transceiver 108 a generates atransmit symbol stream or signal 114T from input symbol stream 112T, andtransmits the symbol stream to corresponding transceiver 110 a overcommunication channel 106 a. Transceivers 108 a and 110 a are said to becommunication “link partners,” because they establish a communicationlink between, and then communicate with, each other over communicationchannel 106 a.

Transceiver 108 a receives a receive signal 114R from communicationchannel 106 a. Receive signal 114R may include a transmit signaloriginated from transceiver 110 a. Additionally and/or alternatively,receive signal 114R represents energy from transmit signal 114T that hasbeen reflected back toward transceiver 108 a. Transmit signal 114T maybe reflected from a channel fault 116 in channel 106 a (such as an openor a short in the channel) and/or from an impedance mismatch at acoupling section between channel 106 a and transceiver assembly 104.Such reflected energy represents an “echo.”

Transceiver 108 a receives symbol stream 114R, and derives a corrected,adaptively filtered output stream 118R from symbol stream 114R. Thesystem and method of this invention uses output stream 118R to detectthe existence of cable faults and to determine their location along thecable.

FIG. 2 is a block diagram of an example arrangement of one transceiver108. Transceiver 108 includes a transmit path 202, a line interface, orhybrid, 204, and a receive path 206. Transmit path 202 includes aDigital/Analog Converter (TxDAC) 210. Transmit path receives an inputfrom a Transmit Physical Coding Sublayer (TxPCS) 208. Together, PCS 208and DAC 210 generate the transmit symbol stream or signal 114T frominput symbol stream 112T. Hybrid 204 couples transmit signal 114T tocommunication cable 106. Also, transmit path 202 provides a signal 213representative of transmit symbol stream 114T to a fault detectioncircuit 218. Signal 213 is a digital representation of transmit signal114T.

Hybrid 204 couples receive signal 114R to receive path 206. Receive path206 includes a high pass filter (HPF) 212, a programmable gain amplifier(PGA) 214, and an analog-to-digital converter (ADC) 216.

Fault detection circuit 218 is coupled between the transmit path 202 andreceive path 206. Fault detection circuit 218 includes a delay line 220,which receives input signal 213 from transmit path 202. An adaptivefilter/echo canceller (EC) 222 is coupled to delay line 220. The ECoutput is multiplied with an EC Gain signal 224 in a multiplier 226. Themultiplied EC output is applied as one input to a summation node 228.Receive signal R[n] outputted from ADC 216 in receive path 206 isapplied as a second input to summation node 228.

FIG. 3 shows a detailed structure of Echo Canceller 222. EC 222comprises a Least Mean Square (LMS), or other suitable algorithm,adapter 304, a series of delay registers 306 a-306 n, a correspondingseries of multipliers 308 a-308 n, and a summer 310. The output ofsummer 310 is modified by a gain G in multiplier 226 and the output ofmultiplier 226 is applied as one input to summation node 228.

The received signal R[n] comprises a series of discrete sampled timesignals. R[n] is the sampled output of ADC 216, indexed by time periods“n”. ADC 216 samples the reflection from the receive channel and outputsR[n]. The output of node 228 is an error signal Error[n]. Error[n] isfed back to LMS adapter 304 of Echo Canceller 222, where it is combinedwith outputs from delay registers 306 a-306 n. The output of LMS adapter304 is then combined in multipliers 308 a-308 n with the TX signal fromtransmit path 202 via signal path 213, delayed in discrete increments“n” by delay registers 306 a-306 n. Each multiplier (for example,multiplier 308 i) has an associated filter coefficient (for example,filter coefficient W_(n)[i]) that adapts itself to the characteristicsof the received data stream such that the filter will converge itself tothe same channel Echo response to the transmitted signals. The combinedsignals are summed in summer 310. The output of summer 310 is combinedin multiplier 226 with a fixed gain factor G. The output of multiplier226 is then applied to summation node 228, where it is subtracted fromreceived signal R[n]. By adapting the EC 222 to realize an approximationof the echo path, a replica of the echo is obtained at the filteroutput, and the converged filter coefficients reflect the actual echoresponse of the communication channel. This replica is subtracted fromthe in-coming signals. The echo response is well defined for a normalchannel, and thus the converged filter coefficients are alsopredictable.

The output of summation node 228RX data−ECdata=Δ,where Δ constitutes the error signals for the Echo canceller's output.

If the gain of the PGA is changed, the RX data and the Error output fromthe summer are affected. All of the variations that are introduced bythe transmit DAC, the hybrid, the analog filter, the PGA and thereceived ADC will affect the actual Echo Canceller's convergedcoefficients. The threshold setting that are used for faults detectionmust account for all of these variations.

The methods described below use the series/set of filter coefficientsmentioned above. According to one aspect of the present invention, thesefilter coefficients adapt to channel conditions, and when interpretedproperly, provide valuable information about the channel, includingspecifically the detection and location of cable faults.

In the case of a fault, due to the mismatch of impedance of the channel,the reflection will be larger than the normal cable reflection. Thiswill cause some tap coefficients to converge to higher values. Thechannel diagnostic system will detect the cable faults by identifyingthose tap coefficients that exceed their normal converged values.

The embodiments of the invention provide a method and systematicapproach for setting up a set of filter coefficient thresholds duringthe channel diagnostic process. First, an optimal threshold is set foreach tap of the filter based on the exponential behavior of thereflection due to the channel fault. The exponential function of thethreshold for each tap can be well defined and estimated fromtransmission line theory and characteristics of the channel. Incontrast, previous threshold setting methods used trial and error basedon the measurement of the reflection, and their staircase approach was avery rough estimation of the exponential behavior of the reflection.

Previous threshold setting methods required adjustment for eachthreshold setting. Depending on how many thresholds are used in thepresent invention (usually at least 10-15 threshold settings to havereliable detection), it is very time consuming to adjust all thethresholds and their sensitivity and variations to the system used inthe measurement could be an issue.

Another aspect of the present invention only requires setting up theinitial value of the threshold at the starting tap. The threshold ofeach following tap can be computed based on the starting tap value. Thismakes the threshold setting a lot easier and a very optimal thresholdsetting can be achieved that maximizes the noise margin for detection.This is in contrast to previous methods, where one threshold was usedfor a large number of taps (from 10 to 20 taps in general). Thethreshold setting could not be made optimal for each tap, and the noisemargin of the detection was degraded.

The present invention uses piecewise linear fitting, which is a verygood approximation for this exponential behavior of the optimalthreshold setting and it can also be implemented very efficiently in theVLSI design.

Echo Canceller Gain Control

Normally, in a no-fault condition, the echo canceller operates with verylow reflection. Thus, the taps do not have to be sized to contain a verylarge number. The hardware can then be optimized on the assumption thatthe reflection number will not be very high at any of the taps. Thisreduces the hardware requirements of the echo canceller. However, if theecho canceller is going to be used to also detect faults, the echocanceller has to be able to accept larger numbers for the higherreflection values that faults cause. To scale the echo canceller up insize to accept the higher numbers requires significantly more hardwarethan is required for normal operation. Because of the infrequency withwhich the cable diagnostics is run, it is preferred to minimize extrahardware with its increased cost and overhead.

If it is known in advance that the system will be looking to detectlarge reflections indicative of a fault, the reflections can bepre-divided before they get to the echo canceller. The value received bythe echo canceller will be a fraction of the actual value and will bewithin the normal operating range of the echo canceller. One would notwant to do this division in normal operation, because the reflectionvalues would be then too low to be useful to the echo canceller.However, in fault detection mode, the large reflections are pre-scaledso that they fit within the dynamic range of the echo canceller.

In normal operation, the reflections that come back from a good cableare small. For this reason, the echo canceller has a relatively smalldynamic range. Because of this, a large fault reflection will be outsidethe range of the EC and will return an error signal because of tapoverflow. The output of the EC will then appear as a normal reflection.The way to overcome this problem is to scale the fault reflection downto a value that fits within the range of the echo canceller.

In one example, the original output of the EC is:${y(n)} = {\sum\limits_{i = 0}^{N - 1}{{W_{n}\lbrack i\rbrack}*{x\left\lbrack {n - i} \right\rbrack}}}$

where w_(n)(i) are the filter coefficients and x(n−i) are the delayedtap inputs, which include the current and previous transmitted data overthe channel. The adaptive algorithm will take the Error[n] to convergethe w_(n)(i).

Error[n] is computed as follows:Error[n]=R[n]−y(n)

where R[n] is the ADC output of the received data from the same channel.R[n]=rd1(n)+rd2(n)+additive noise.

The additive noise may be assumed to be Additive White Gaussian Noise(AWGN) with zero mean. This will contribute some small tap noise for theconverged filter coefficients, and may be ignored to simplify thederivation. rd1(n) is the echo from the same channel (cross-talk betweenthe channels is ignored because during the channel diagnostic process,data is sent only for the current channel to be diagnosed).

In one example, the echo response of the channel can be modeled as alinear finite impulse response (FIR) filter as:${{d1}(n)} = {\sum\limits_{i = 1}^{N - 1}{{h(i)}*{x\left( {n - i} \right)}}}$

h(i) are the echo impulse response. rd2(n) is the received signalscoming from another channel connected to the cable, which can be a linkpulse, or other data traffic. So the Error[n] will be:${{Error}\lbrack n\rbrack} = {{\sum\limits_{i = 0}^{N - 1}{\left\lbrack {{h(i)} - {w(i)}} \right\rbrack*{x\left( {n - i} \right)}}} + {{{rd2}(n)}.}}$

In one example, the actual adaptation algorithm uses the mean-squareerror (MSE) as the performance function to converge the filtercoefficients. There is no correlation (or very minor correlation)between x(n) and rd2(n). Therefore one can simply takeE[x(n-i)*rd2(n)]=0 as a good approximation, and the result of the MSE isthe following (E[.] denotes statistical expectation): $\begin{matrix}{{MSE} = {E\left\lbrack {{{{Error}\lbrack n\rbrack}}\hat{}2} \right\rbrack}} \\{= {{E\left\lbrack {{{\sum\limits_{i = 1}^{N - 1}{\left\lbrack {{h(i)} - {w(i)}} \right\rbrack*{x\left( {n - i} \right)}}}}\hat{}2} \right\rbrack} + {{E\left\lbrack {{{{rd2}(n)}}\hat{}2} \right\rbrack}.}}}\end{matrix}$

To determine an optimal w_(n)(i) that will minimize the MSE, a partialderivative of the MSE is applied with respect to every tap weight (thecoefficients of each tap) and this derivative is set to zero, whichgives:w(i)=h(i) for i=0, 1, . . . , N−1.

This shows that the converged value of the EC will be a good estimationof the real channel echo response. However, in the case of overflowduring the convergence of w(i) for the reasons discussed above, somew(i) may be quite different from its optimal value h(i) that it issupposed to converge.

In one embodiment, a gain tap is introduced at the output of the EC toincrease the dynamic range of EC coefficient range. By adding a digitalgain tap at the output of the EC, the Error[n] is: $\begin{matrix}{{{{Error}\lbrack n\rbrack} = {{R\lbrack n\rbrack} - {G*{y(n)}}}},{{where}\quad G\quad{is}\quad a\quad{gain}\quad{{tap}.}}} \\{{{Error}\lbrack n\rbrack} = {{R\lbrack n\rbrack} - {G*{\sum\limits_{i = 0}^{N - 1}{{w(i)}*{x\left( {n - 1} \right)}}}}}}\end{matrix}$

It can also be written as: $\begin{matrix}{{{Error}\lbrack n\rbrack} = {{R\lbrack n\rbrack} - {\sum\limits_{i = 0}^{N - 1}{G*{w(i)}*{x\left( {n - i} \right)}}}}} \\{= {{R\lbrack n\rbrack} - {\sum\limits_{i = 0}^{N - 1}{{w^{\prime}(i)}*{x\left( {n - i} \right)}}}}}\end{matrix}$

where w′(i)=G*w(i) and the same MSE equation with this substitution ofw(i): $\begin{matrix}{{MSE} = {E\left\lbrack {{{{Error}\lbrack n\rbrack}}\hat{}2} \right\rbrack}} \\{= {{E\left\lbrack {{{\sum\limits_{i = 0}^{N - 1}{\left\lbrack {{h(i)} - {w^{\prime}(i)}} \right\rbrack*{x\left( {n - i} \right)}}}}\hat{}2} \right\rbrack} + {{E\left\lbrack {{{{rd2}(n)}}\hat{}2} \right\rbrack}.}}}\end{matrix}$

so the optimal converged value of w′(i) will be:w′(i)=G*w(i)=h(i).

and the actual EC coefficients w(i) will converge to their optimalvalue:w(i)=h(i)/G.

The gain G can be programmed to be 2^(k) (k=0, 1, 2, . . . ) to simplifyimplementation.k=0: w(i)=h(i);k=1: w(i)=h(i)/2;k=2: w(i)=h(i)/4.

Since the gain tap G can have the form of 2^(k), the actualimplementation of the digital multiplication is an arithmetic shift. Bymaking G=2 or 4, more dynamic range is possible for w(i) to convergewith the same word-length implementation, and the overflow issue can besolved.

The echo canceller includes a set of coefficients that will converge tothe channel's echo impulse response. By comparing this set of filtercoefficients with a predetermined set of filter coefficient thresholds,channel faults that add additional reflection due to channel impedancemismatch will be detected.

The above embodiments and examples describe a solution for the overflowissue that could occur during the convergence of the adaptive filterused for a channel diagnostic system. By introducing a gain tap at theoutput of the EC filter, a dynamic convergence range of the filtercoefficients is increased without increasing the fixed-point word lengthof the coefficients that is already optimal for the normal datacommunication.

The above embodiments and examples apply an original EC filter design tothe cable diagnostic system without changing the word length of thefilter coefficients and the corresponding arithmetic logic. Adding thegain tap at the output of the EC filter might add more noise (some noiseenhancement and also the increased quantization noise due to the gaintap) during the convergence of the filter coefficients, which will addsome tap noise to the final converged coefficient values. However, sincethe system is only required to detect any reflections due to the channelfaults, good noise margin is possible for the detection as long as thereis no overflow during the convergence of the EC.

There are generally three main types of cable faults. These include asevered connection or “open circuit,” a fused connection or “shortcircuit,” and a fused connection between two channels or “short betweenpairs.” Open circuit and short circuit faults are collectively referredto as “open/short” faults. The present invention relates in part todetermining whether a fault has occurred, and, if so, whether the faultis an open/short or a short between pairs. Another aspect of theinvention relates to determining the location of the fault along thecable. Still another aspect of the invention relates to determiningcable length for a good cable (that is, a cable that does not exhibit afault condition).

A cable of the type to which this invention is directed can beconsidered to be similar to a transmission line. Therefore, theproperties of transmission line theory, especially those concerningreflection, apply to these cables. Signals transmitted through the cablewill be subject to at least minimal reflection as a result of inherentmismatch between the cable and the termination, either at another cableconnection or another connector. The reflection can be detected on thereceive path 206 of transceiver 108. A fault will generally be detectedas a larger reflection than a reflection from a good cable.

Cable diagnostics are typically undertaken when a problem is detectedwith a channel or cable as a whole. At that time, the user (usually thesystem administrator) will be alerted to initiate a diagnostic sequenceto determine whether there is a fault in the cable. This is done bysending a test signal through the cable and detecting the reflectionthat is returned.

The existence of a fault in a cable can be determined by comparing thefilter coefficients at each tap to a predetermined set of filtercoefficient fault thresholds that set a limit on reasonable values ofthe filter coefficients corresponding to non-fault channel conditions.An abnormally enlarged filter coefficient that exceeds one suchpredetermined threshold indicates a fault in the channel. Also, a goodapproximation of the distance between the filter (in the transceiver)and such an indicated channel fault can be calculated based on the tapnumber corresponding to the enlarged coefficient. In addition, when nochannel fault exists, a good approximation of the channel length can bedetermined by comparing the filter coefficients to a different set ofpredetermined thresholds indicative of channel length when no channelfault exists, and finding the tap number of any coefficient that exceedsone of these thresholds.

The filter coefficients exceed the above mentioned thresholds for thefollowing reason. Channel faults (and similarly, the coupling betweenthe end of the channel and the remote transceiver or link partner) causean impedance mismatch in the channel. Thus, the fault (or channel-endcoupling) returns substantial reflections of the transmitted signal backto the receive path 206. The adaptive filter attempts to remove theeffects of the reflections or echoes by enlarging some of the filtercoefficients. The longer the distance between the transceiver (forexample, transceiver 108 a) and the channel fault, the longer it takesfor the reflection to return to the adaptive filter, and the higher thetap number(s) of the enlarged coefficients. Also, the longer thedistance between the transceiver and the channel fault or coupling, thesmaller the amplitude of the reflected signal, and the less thecorresponding filter coefficient values are enlarged.

Another way of looking at this process is as follows. Under normalconditions, the adaptive filter receives the transmitted signal from itsassociated transmitter, as the line interface initially couples thetransmitted signal to the channel. By storing delayed copies of thetransmitted signal in the filter delay elements, and adjusting thecorresponding coefficient values, the adaptive filter adjusts to the“timing” of the transmitted signal in the channel. Such timing of thetransmitted signal is inherently represented by the adapted filtercoefficients.

With regard to channel length determinations, when an echo of thetransmitted signal arrives at the adaptive filter relatively soon afterthe initial transmission of the transmitted signal, one or more of therelatively earlier filter coefficients (that is, filter coefficientscorresponding to smaller tap numbers) will self-adjust to a largercoefficient value, that exceeds its respective threshold. The methodsdescribed below will then report that the cable is relatively shortbecause of the “soon after” arrival of the echo.

On the other hand, when an echo of the transmitted signal arrives at theadaptive filter a relatively long time after the initial transmission ofthe transmitted signal, then one or more of the relatively later (highertap number) filter coefficients will have a higher-than-normal valuethat exceeds the respective threshold, and the method will report arelatively longer cable length due to the later arrival of the echo.

Open/Short Fault Detection

Fault testing is done at the option of the user. System diagnostics canbe run periodically or aperiodically, as needed by the user, typicallythe system administrator. The system can also be set to run faulttesting diagnostics automatically when the system attempts to establisha connection or link to transfer data and is unable to do so. The systemautomatically assumes that there is a fault and will switch intoauto-detection mode.

A first set of thresholds indicative of channel faults (i.e., open/shortthresholds) is used to determine the existence of and distance to achannel fault, while a possibly different second set of thresholdsindicative of channel length (i.e., length thresholds) is used todetermine a channel length when no channel fault has been detected. Dueto the nature of adaptive filter coefficientsw[i], poor results may beobtained if all of the filter coefficients w[i] are compared to a singlethreshold. This is due to the fact that filter coefficients w[i]corresponding to larger tap numbers (and thus, larger time delay) tendto have smaller absolute values after they are properly converged. Thus,a set of thresholds including thresholds of different values is used inthe present invention. In setting threshold values, one must take intoconsideration the values of potential fault reflections. If thethreshold is set too low or too high, the value of the fault reflectioncould produce an error reading. If the threshold is set too low, somesmall reflection due to imperfect termination or some variations due tothe return loss of the cable plus any noise with the converged ECcoefficients will be larger than this too low threshold and will resultin a false detection. Conversely, if the threshold is set too high, itwould miss detecting lower value fault reflections due to the distancefrom the signal source or due to the fact that the fault is caused by ashort between wires (a “short between pairs”), which generally produceslower value faults than open/short faults.

In a prior system, a staircase approach was used to determine theopen/short thresholds and length thresholds. Filter coefficients w(i)were divided into three or four groups, and each group had its owncorresponding fixed threshold for open/short thresholds and lengththresholds, with each group being somewhat smaller than the thresholdfor the previous group. Thus, the different threshold values followed astaircase of values. The number of groups was related to the number ofregisters available to store the coefficients. Cost and spacerestrictions limited the number of groups to three or four.

This prior system of staircase groups of filter coefficients produced aless than satisfactory response to fault detection. Each group covered alarge number of taps. For example, in a typical system with 190 tapscovering a 150 m cable, each of the four groups would have to span 47-48taps over a distance of 37.5 m per group. Given the fall off ofreflection amplitude over distance, the reflections returning from thefurthest taps in a group are significantly smaller than reflectionsreturning from the nearest taps in that group. A threshold value thatworks for the closer taps may well miss faults from the farther taps.

Theoretically, the threshold should follow cable attenuation. Accordingto standard transmission-line theory, it is possible to compute howattenuation changes with cable length. The amplitude of the channelreflection is an exponential decay function of cable length, which is alinear function of the tap number of the echo canceller. The ECcoefficients, which are the estimation of the channel reflection, shouldalso follow this exponential function with the tap number. Thisexponential function can be described as follows:Threshold=A*exp(−B*tap)

where A and B are constants that depend on the particular cableproperties and are empirically determined based on fundamentaltransmission line theory.

Ideally, one would like to have one threshold for each tap. However,this becomes very expensive, because each threshold requires a registerto store the threshold value. This becomes expensive from both ahardware cost and a “real estate” cost viewpoint. Therefore, a way mustbe found to cost-effectively use fewer registers while still obtainingreasonably accurate readings.

The present invention solves the problem by using a piecewise linearfunction (PWL) to approximate the exponential change in threshold withcable length. Using a piecewise linear function, it is only necessary tostore two values for each cable segment. This is a considerableimprovement over the stepwise arrangement which required one registerfor each step. Using the piecewise linear function, only two registersare required for each cable segment, one to store each end value neededto compute the line. Using the piecewise linear function, one can getvery close to a good approximation at each tap by dividing the wholecable length into a few segments (i.e. 4 segments to cover 0-150 m) andusing a linear approximation for each segment of cable

Referring to the graph of threshold settings shown in FIG. 4, theequation used is y=a*b+c.

1. For cable length estimationlength=a ₁*tap number+c ₁

Thus, cable length is a linear function of the tap number.

2. For threshold (to compute PWL)threshold=tap a _((i))*tap number+tap b(i)

-   -   where i=0, 1, 2, 3 . . . , n

The same hardware, an example of which is shown in FIG. 5, discussedbelow, is used to estimate length and threshold. The controller controlsthe input multiplexers (MUXes) to select the inputs to the multiplierand adder. The inputs come from the memory registers (not shown) wherethe threshold values are stored. In a first time window, the thresholdwill be calculated, and in a second time window the length will becomputed.

Sequentially, the process starts from tap 0. At each tap for the currentcable segment, the same linear parameters a_(i) and b_(i) for that tapare selected. This will set the threshold at that tap for the currentsegment. Then the threshold for that tap is compared to the reflectedsignal. If the reflected signal is greater than the threshold, this isan indication of a fault. It is also possible to determine the end ofthe cable by this method. Due to mismatches in cable and terminationimpedances, a termination appears as a fault and sends back a smallreflection that will exceed the thresholds that are predetermined for thecable length detection.

The simplified algorithm is as follows:

-   -   1. Compute the threshold at each tap using PWL.    -   2. Use the computed threshold to detect an open/short (i.e.,        compare the threshold to the tap weight or coefficient (w(i))).        For each tap number, there is a corresponding tap weight.    -   3. If w(I is less than the threshold at the tap, then terminate.    -   4. If w(i) is greater than the tap threshold, then stop        searching and compute the tap length.

In an example implementation, a cable includes four pairs of wires. Thisdiagnostic process is performed for each pair of wires of the four-wirepair in the cable, because there could be a break in one of the pairsbut not all of them.

The process for comparing EC coefficients to thresholds must be doneover several taps, for example, 5-10 taps. Due to dispersion, it may benecessary to take a reading from several adjacent taps to get anaccurate determination of whether there is a real peak or merely anaberration.

A good approximation and efficient implementation in ASIC design of thisexponential function uses PWL functions to approximate the computationof the exponential function in different regions of taps. For each groupof taps, the following linear function is used:Threshold(tap)=A(i)*tap+B(i) (i=0, 1, . . . N)where N is the number of groups of taps and A(i) and B(i) can be foundby using a linear fitting method. A different linear function is used toestimate the thresholds for each group. In contrast, in conventionalmethods each threshold had to be determined and stored separately. Or,as in the previous system described above, a small group of thresholdswas used for a large group of taps. Using the method according to theabove embodiments and examples, many thresholds can be represented byonly two parameters.

In one example using a full duplex system, inaccurate hybrid echocancellation can cause a large deviation of the threshold setting forthe taps between 0-10. To circumvent this practical issue, a constantthreshold setting threshold 0 is added for taps 0-10 instead of usingPWL approximation.

FIG. 5 shows the resource sharing arrangement for computing piecewiselinear function (PWL) threshold and cable length. One of the features ofthe present invention is the efficient use of hardware to minimize theamount of components that are required to perform the desired functions.

A linear function can be used to estimate cable length. Two constantsare needed: C_(len) _(—) _(linear) and C_(len) _(—) _(Const). These areeach programmable parameters. The PWL function is used to determine theoptimal threshold for each tap. Theoretically, the optimal thresholddetermination should be an exponential function. However, it is not costeffective to implement an exponential function in the availablehardware. Therefore the present invention makes use of PWL toapproximate the exponential function.

A controller controls in real time when the cable length for a brokencable must be computed and when the thresholds must be computed. UsingPWL, the threshold for each tap is first computed. Then the reflectionsare compared to the computed thresholds. If a cable fault is detected,then the cable length is computed to determine the location of the faultalong the cable. The data path is used to compute the threshold for eachtap. The thresholds for each tap are computed using PWL and looking atsegments. For each segment there is a different A(n) and B(n). A and Bare programmable parameters which are used to compute the PWL of thethreshold.

Referring to FIG. 5, a first multiplexer (MUX) 502 selects predeterminedprogrammable parameters A(0)-A(3). A second multiplexer (MUX) 504selects predetermined programmable parameters B(0)-B(3). Depending onthe tap number to be calculated, any one of A(0)-A(3) and any one ofB(0)-B(3) are selected. The taps are divided into four groups. For eachgroup, a different A(i) and B(i) parameter are selected. The selected Aparameter is combined in a multiplexer 506 with a predeterminedprogrammable parameter C_(len) _(—) _(Linear). A selected B(i) parameteris combined in a multiplexer 508 with a predetermined programmableparameter C_(len) _(—) _(Const). The output of MUX 506 is connected toone input of a multiplier 510. The second input to multiplier 510 comesfrom a finite state machine (FSM 512. The outputs of multiplier 510 andMUX 508 are applied as inputs to an adder 514. Adder 514 generates atits output:C _(len) =C _(len) _(—) _(Linear)*tap+C _(len) _(—) _(Const)andThreshold_(—) PWL=A[i]*tap+B[i] (i=0,1,2,3)

In one example, A(i) and B(i) are programmable parameters. They aremultiplexed to the input of the multiplier and adder during the channeldiagnostics depending on the current tap number and which group this tapbelongs to. By doing that, resource-sharing of the same multiplier andadder combined is possible, which makes the ASIC design very efficient.Furthermore, this combined multiplier and adder is also used to computethe cable length polynomial for reporting the length to a cable fault.To achieve this optimization of logic design, time-sharing is used toschedule the resource sharing depending on the state of the cablediagnostic state-machine.

The parameters of A(i) and B(i) estimated from the PWL method usuallyrequire more than 8-bits to represent and they are also signed numbers.In order to fit this unsigned 8×8 multiplier and 17-bit adder structure,a two-step scaling method is used. The sign of A(i) and B(i) (A(i)<0,B(i)>0) is changed when programming these two parameters for the system.The sign of the final output of the multiplier and adder output is theninverted. This is a very efficient way to implement the arithmetics forboth PWL threshold computation as well as cable length computation bysharing the same multiplier and adder logic.

In an example operational scenario, transceivers 108 a and 110 a attemptto establish a valid communication link between themselves, to enablecommunication over the valid link (and communication channel 106 a). Todo this, transceivers 108 a and 110 a traverse a communication linksetup protocol. Typically, this includes exchanging initial hand-shakingsignals, and verifying a valid link has been established, as would beapparent to one skilled in the relevant art(s).

When a valid link exists, filter coefficients w(0)-w(N−1) (collectivelyreferred to as filter coefficients w(i)) have properly converged tosettled coefficient values. That is, filter coefficients w(i) have hadsufficient time to adapt to, and therefore indicate, the characteristicsand/or conditions of communication channel 106 a. Thus, convergedcoefficients provide useful information about communication channel 106a.

A valid link may not exist between link partner transceivers 108 a and110 a. For example, there may be a fault, such as a break, incommunication channel 106 a, or transceiver link partner 110 a may notbe connected to the communication channel. When a valid link does notexist, filter coefficients w(i) can be in an indeterminate(non-converged) state, thus providing little or no useful informationabout channel 106 a.

When no valid link is detected, the user will initiate the diagnosticsequence. In this case, a signal is transmitted over transmit path 202.A copy of the transmitted signal is sent to fault detection circuit 218.The transmitted signal is delayed in registers 306 corresponding to thetaps in the cable.

The received signal R[n] is a series of discrete sampled time signals.R[n] is the sampled output of ADC 216, indexed by time periods “n”. ADC216 samples the reflection from the receive channel and outputs R[n].Signal R[n] comprises one input of summation node 302. The output ofnode 302 is error signal Error[n]. Error signal Error[n] is fed back toLMS adapter 304 of Echo Canceller 222, where it is combined with outputsfrom delay registers 306 a-306 n. The output of LMS adapter 304 is thencombined with the outputs from delay registers 306 a-306 n and summed insummer 310. The output of summer 310 is combined in multiplier 226 witha fixed gain factor G. The output of multiplier 226 is then applied tosummation node 302 where it is subtracted from received signal R[n].

The EC tries to estimate the channel reflection. If the EC estimation isperfectly aligned with R[n], thenError[n]=0.

The error signal Error[n] supplies an input to the LMS algorithm of theEC to adjust the coefficients of the filter to perform channel modelingor channel estimation, according to the following equation:Echo output Y(n)*G→R[n]

-   -   where Y[n] is the output of EC 222 and G is a programmable fixed        gain value.

Initially, it is not known what the actual echo response will be.Therefore, the coefficients of the filters may all be zero. At thispoint, Y(n)=0, and the echo output will be zero. The error will be largebecauseR[n]−Echo output=R[n]=Error[n].

Initially, therefore, Error[n] will equal R[n] and be large. Error[n] isapplied to the LMS adapter 304 and the filter coefficients are adjustedaccording to the LMS algorithm. The filters combine the LMS adapteroutput (i.e., Error[n]) with reference signals (i.e., the transmitsignals) to produce updated filter coefficients. The echo response atdifferent cable lengths is equivalently modeled by the filtercoefficients w(0) -w(n−1). This provides the profile of the channel Echoresponse. After sufficient iterations, the echo coefficients willconverge so that the Echo Canceller will generate a good estimation ofthe real time reflection of the transmitted signals and the error signalwill approach zero.

Each filter coefficient w(i) represents the return from different pointson the cable. In this way, it is possible to determine the location of afault when the reflected signal falls outside the normal coefficientboundaries.

One reason for using EC 222 in cable diagnostics is to reduce Error[n]close to zero. Different cable conditions will produce different echoresponses. EC 222 tries to estimate the echo responses by comparing itsoutput with the ADC sample output R[n] (the digitized reflection signalscorresponding to the delayed cable echo response). EC 222 uses thedifference between the ADC sample and the EC output as an error signalto adapt the coefficient of the filter. The gain G is chosen to be avalue that will avoid overflow issues. Typically, G can have a value of2 or 4, although any appropriate value can be used, as would be apparentto one skilled in the relevant arts.

To determine a fault in the cable, it is sufficient to compare theconverged reflection coefficient with a known reference for a good cableat each tap along the cable. This determines the threshold at each tap.

It may be useful to determine cable length. A complete cable may be madeup of multiple sections. For example, one section runs from the computerto the wall tap. A second section runs from the wall tap to a router. Athird section may run from the router to a server. By knowing cablelength, one can determine which connector may be bad, assuming it is oneof the connectors that is bad. Then it may be only necessary to repairthe connector instead of having to replace the entire cable.Alternatively, if one segment of the cable is bad, then by knowing thelength of the cable at the break point, it is possible to merely replacethat segment instead of replacing the entire cable.

Even good cables with no faults generate reflections at theirtermination. These reflections can be detected by searching from thelast tap for any coefficients that are above the predetermined thresholdsettings for good cable length detection, and the cable length can bedetermined from that measurement.

Short Between Pairs Fault Detection

A cable of the type to which this invention is directed generallycomprises four pairs of twisted wires. If the wires of a given pair arebroken, a relatively large reflection will occur. Alternatively, if thewires of a given pair are shorted to each other, a similar, relativelylarge reflection will occur. However, if a wire of one pair is shortedto a wire of a second pair, a much smaller reflection will occur,although that reflection will still be larger than the normal residualreflection of a good cable. In general, the short between pairsreflection will be about 25% smaller in value than an open/shortreflection. It is desirable to detect a short between pairs that couldaffect operation of the cable.

One way to be able to detect a short between pairs is to lower thethreshold to a value below that of the short between pairs reflection.One of the problems associated with using a low threshold is what isknown as “tap noise”. Tap noise comprises variations in reflections dueto different characteristics of cables and different gain settings.

For large reflections it is desirable to set a high threshold to reducethe effect of path noise and low level reflections. The equation is:C _(b)(i)±ΔC ₁>threshold>C _(g)(i)±ΔC ₂

ΔC₁ and ΔC₂ are functions of the conditions of the cables. ΔC₁ is acoefficient tap noise when the cable is broken and ΔC₂ is a coefficienttap noise when the cable is good. These two coefficients may bedifferent. These are functions of the reflection in the cables. Theoptimum threshold setting is accomplished by using different settingsfor a simple open/short and for a short between pairs. The samereflection value occurs whether the pair is open or the pair is shorted(within a pair).

The process proceeds as follows.

1. First examine each pair of wires of the cable using the highthreshold detection to determine if there is a simple open/short.

2. Once an open/short is found the process stops looking for additionalopen/shorts.

3. If no open/short is found, the threshold level is changed (lowered)and the gain of PGA 214 is increased to check for a short between pairs.

4. To check for a simple open/short it is only necessary to look at bothwires of a single pair at any one time.

5. To check for a short between pairs it is necessary to check two pairsat a time.

Because the threshold level has to be set very low when checking forshorts between pairs, there is a lower level of confidence that a shortwill be detected. False readings are more likely to occur when lookingfor shorts between pairs because the reflection of the short may bemixed with noise levels. Therefore multiple readings should be taken toraise the confidence level that a short has been detected. Thereflection in a short between pairs situation should normally be higherthan normal noise, but this is not always the case.

The confidence level can be raised by detecting the signature of thereflection. FIGS. 6 and 7 show an open reflection signature and a shortreflection signature, respectively. The portion 602 of the wave 604below the axis in the open reflection (FIG. 6) and the portion 702 ofthe wave 704 above the axis in the short reflection (FIG. 7) are calledthe tails. These are the signature portions of the reflection. Areflection from a termination or from normal line noise will not havethe signature tails of a reflection from an open/short.

The combined process for detecting open/short faults and shorts betweencables is described below.

FIG. 8 is a flow chart showing the combined process for determiningopen/short faults and short between pairs faults. Each cable comprises 4twisted pairs of wires. A short between pairs means that one wire of onepair is shorted to one wire of a second pair.

A goal is to optimize the EC gain and gain setting of the PGA for thefirst pass to detect open/short faults and then for the second pass todetect only a short between pairs. In step 804, the EC gain and AGC areprogrammed to set the optimal gain for the PGA. In step 806, aftersetting the gain, “idle” data is sent through each twisted cable pair todetect the reflection. The EC is an adaptive filter coupled to each ofthe four channels in the cable. The EC knows the data sent to thechannel pairs. The EC looks at the sampled reflection data coming backfrom each pair after the data is processed by the PGA and ADC. Bylooking at the sampled reflections, the EC can adapt itself so that thefilter coefficient will be matched with the reflections from each tapalong the cable. At the same time, the EC will adapt and converge to thechannel's response to the reflection.

The cable can be thought of as a tapped delay line. The tapped delayline is modeled as a finite impulse response (FIR) filter. The EC isalso modeled as an FIR filter. Each tap coefficient corresponds to thereflection from the corresponding tap. The echo will converge so that itmimics the echo response from the same cable. Therefore, afterconvergence the whole characteristic of the reflection of the cable isknown. At step 808, the echo response is read. What this means is thatit is the echo coefficient of each pair that is read. A search isperformed to determine whether an open/short exists. A search foropen/short means that each tap coefficient is compared to apredetermined threshold. If the tap coefficient is greater than thethreshold, this is indicative of an open/short fault. A positivecoefficient sign is indicative of an open fault. If the sign of thecoefficient is negative, this means that the fault is a short within apair.

At step 810 the results of the first pass search are stored in theinternal registers. At step 812, the EC gain and AGC of the PGA areoptimized to a new set of coefficients having a lower threshold than thecoefficients of the first pass to enable detection of either a shortbetween pairs or channel termination. In both cases, the reflection willbe much lower than the reflection from an open/short fault. If the gainthreshold is set too high, that is, it is kept at the same level as thefirst pass, the reflection will be below this threshold and the systemwill not detect a short between pairs or a channel termination.

A second test signal is then sent along the cable into each pair ofwires and the reflections are converged at step 814 to read the echoresponse in step 816.

Step 818 is a decision point. If the open/short search at step 808 hasdetected an open/short fault, then there is no need to search for ashort between pairs. In that case, the process proceeds directly to step824 to determine the unbroken cable length, that is, the length of thecable from its start point up to the location of the open/short fault.This is determined by determining the tap number at which the tapcoefficient is above the first predetermined threshold, representing thedetected open/short fault. At the same time, the system will determinethe total cable length, based on detection of reflections from the good(non-faulty) cable pairs. A report of the diagnostic findings isreported to the user at step 826 and the process finishes at step 828.

If no open/short fault was detected on the first pass, then the processproceeds to step 820 to search for a short between pairs based on theresults of the second pass. The results of both the first and secondpasses are then combined at step 822. The non-broken cable length isthen determined at step 824. If no fault has been detected, the resultwill be a report of a good cable and its length. If a fault has beendetected, the diagnostic system will issue a report indicating the totalcable length and the type and location of the fault.

It should be noted that the algorithms for determining total cablelength and fault location are different. The algorithm for detecting acable fault starts with tap 0 and proceeds forward to tap N. In oneexample, N=190. However, it will be apparent to one skilled in therelevant art that N can be any suitable number, depending on suchfactors as the length of the cable and the accuracy of measurement thatis desired. The algorithm for detecting total cable length, on the otherhand, proceeds back to the source from tap N.

Another issue that needs to be considered is the fact that, whenmeasuring cable length, the reflection will be quite small. PGA 214 musttherefore be adjusted to allow measurement of the reflection over thetap noise. If the signal is small, the echo canceller may not be able toadapt sufficiently to measure a small reflection such as occurs withcable length detection and short between pairs faults.

The embodiments of this invention can use VLSI micro-architecture designto implement the PWL approximation algorithm and resource-sharing withthe similar arithmetic computation of cable length polynomial.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample, and not limitation. It will be apparent to persons skilled inthe relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.

The present invention has been described above with the aid offunctional building blocks and method steps illustrating the performanceof specified functions and relationships thereof. The boundaries ofthese functional building blocks and method steps have been arbitrarilydefined herein for the convenience of the description. Alternateboundaries can be defined so long as the specified functions andrelationships thereof are appropriately performed. Any such alternateboundaries are thus within the scope and spirit of the claimedinvention. One skilled in the art will recognize that these functionalbuilding blocks and modules can be implemented by discrete componentsincluding digital and/or analog circuits, application specificintegrated circuits, processors executing appropriate software,hardware, firmware and the like or any combination thereof. Thus, thebreadth and scope of the present invention should not be limited by anyof the above-described exemplary embodiments, but should be defined onlyin accordance with the following claims and their equivalents.

1. A method for detecting faults in cables, comprising: a) receiving afirst reflected signal; b) comparing the first reflected signal with afirst threshold; c) if the value of the first reflected signal isgreater than the value of the first threshold, then terminatingdetecting; d) if the value of the first reflected signal is not greaterthan the value of the first threshold, then e) comparing a secondreflected signal different from the first reflected signal with a secondthreshold.
 2. The method of claim 1, further comprising: f) reporting afirst fault if the value of the first reflected signal is greater thanthe first threshold.
 3. The method of claim 2, further comprising: g)reporting a second fault if the value of the first reflected signal isless than the first threshold and the value of the second reflectedsignal is greater than the second threshold.
 4. The method of claim 1,further comprising: f) reporting a second fault if the value of thesecond reflected signal is greater than the second threshold.
 5. Themethod of claim 1, further comprising: f) adding a first receiver gainto the first reflected signal; and g) adding a second receiver gaindifferent from the first receiver gain to the second reflected signal.6. The method of claim 5, further comprising repeating steps a)-e) witha third predetermined receiver gain setting higher than said firstreceiver gain setting.
 7. The method of claim 1, further comprising: h)determining whether said second reflected signal has a signature tailthat signifies a cable fault.
 8. The method of claim 5, wherein thesecond threshold is higher than the first threshold.
 9. A method fordetecting faults in cables, comprising: a) setting a predetermined gainof an echo canceller (EC); b) transmitting a test signal through atleast one pair of a twisted pair cable; c) receiving a reflected signalfrom said at least one twisted pair; d) converging the output of step(c) in the EC; e) comparing the result of step (d) with a knownthreshold; f) determining whether the compared signal is greater inmagnitude than the known threshold and if the compared signal is greaterthan the known threshold, then g) terminating the comparison andreporting a fault in the cable.
 10. The method of claim 9, wherein, ifthe compared signal is greater than the known threshold, then: h)determining whether the sign of the compared signal is positive ornegative, to thereby determine whether the fault comprises an open orbreak in the cable pair, or a short between wires of the cable pair,respectively.
 11. The method of claim 9, further comprising: a) settinga plurality of gains of a programmable gain amplifier (PGA)corresponding to a plurality of taps along the length of the cable; b)combining the gains of the PGA with the reflected signal of step (c); c)converging the result of step (i) for each of the plurality of PGAgains; d) comparing the results of step (j) with a plurality of knownthresholds calculated according to a linear piecewise function; e)determining whether any compared signal is greater in magnitude than thecorresponding known threshold, and if so, then f) determining thelocation of the region of the cable that produced the signal of greatermagnitude than the corresponding known threshold.
 12. The method ofclaim 9, further comprising: g) after step (d), transmitting a secondtest signal through the twisted pair; h) receiving a second reflectedsignal from said twisted pair; i) combining a gain from a programmablegain amplifier (PGA) with the second reflected signal in the PGA; j)converging the output of step (l) in the EC; k) comparing the result ofstep (m) with a second known threshold; l) if the magnitude of thesecond compared signal in step (h) is less than the known threshold,then m) determining whether the second compared signal is greater inmagnitude than the second threshold, and if the second compared signalis greater than the second threshold; n) terminating the comparison andreporting a fault in the cable.
 13. The method of claim 12, furthercomprising: o) setting a plurality of gains in the PGA and ECcorresponding to a plurality of taps along the length of the cable; p)converging the result of step (m) for each of the plurality of PGAgains; q) comparing the results of step (p) with a plurality of knownsecond thresholds calculated according to a linear piecewise function;r) determining whether any compared signal is greater in magnitude thanthe corresponding known threshold, and if so, then s) determining thelocation of the region of the cable that produced the signal of greatermagnitude than the corresponding known threshold.
 14. The method ofclaim 12, further comprising: determining whether said second reflectedsignal has a signature tail that signifies a cable fault.
 15. Apparatusfor detecting faults in cables, comprising: a transmission path fortransmitting a signal along a cable; a receiving path for receiving areflected signal from the cable; an echo canceller located between andcoupled to said transmission path and said receiving path; means forconverging a filter coefficient of the echo canceller as a function ofthe reflected signal; means for comparing the converged coefficient witha predetermined threshold; and means for reporting a fault in the cableif the compared converged coefficient has a magnitude that is greaterthan said threshold.
 16. Apparatus according to claim 15, furthercomprising: a programmable gain amplifier located in said receiving pathfor adding a predetermined gain factor to the reflected signal. 17.Apparatus for detecting faults in cables, comprising: a transmissionpath for transmitting a signal along a cable; a receiving path forreceiving a reflected signal from the cable; a programmable gainamplifier located in said receiving path for adding a predetermined gainfactor to the reflected signal; an echo canceller located between andcoupled to said transmission path and said receiving path; means forconverging a filter coefficient of the echo canceller as a function ofthe reflected signal processed through the programmable gain amplifier;means for comparing the converged coefficient with a predeterminedthreshold; and means for reporting a fault in the cable if the comparedconverged coefficient has a magnitude greater than said threshold. 18.The apparatus of claim 17, further comprising: wherein said receivingpath receives a second reflected signal; wherein said programmable gainamplifier adds a second gain factor to the second reflected signalwherein the converging means converges a second filter coefficient ofthe echo canceller as a function of the second reflected signalprocessed through the programmable gain amplifier; means for comparingthe second converged coefficient with a second predetermined threshold,the value of which is determined using a piecewise linear function; andmeans for reporting a fault in the cable if the second comparedconverged coefficient has a magnitude greater than said secondpredetermined threshold.